This invention relates to an ignition apparatus and more particularly to an ignition apparatus provided with a control circuit for preventing a small oscillation caused at the time when a current is limited.
Usually, an ignition apparatus for an internal combustion engine is provided with a current control circuit for preventing more current than a predetermined level from flowing through a power transistor which is connected in series with the primary winding of an ignition coil to control flow of the primary current of the ignition coil. In such a control circuit a small oscillation is generated at the time the collector current of the power transistor is limited because of a high gain of the control circuit and delay in the operational response of the power transistor. This small oscillation in turn causes the base current, the collector-emitter voltage and the collector current of the power transistor to oscillate. As a result, stability in operation of the power transistor is degraded and the collector loss is increased so that heat generated may destroy the power transistor. For this reason, the oscillation, at the time the collector current is limited should be prevented.
In the prior art measures for preventing such an oscillation, either the gain of the entire current control circuit is limited to a small value, or a capacitor is inserted in the feedback loop of the current control circuit so as to utilize delay in signal phase, as disclosed by the U.S. Pat. No. 3,838,672.
In the case where a part of the current control circuit is constructed in a monolithic IC configuration, however, the current amplification factors of transistors contained in the monolithic IC have different values within a range of 30 to 200 and there is only a little probability that a monolithic IC containing transistors all having small current amplification factors will be obtained. Accordingly, the productivity of monolithic IC's with which the gain of the entire current control circuit can be made small, is limited, so that the yield rate of ignition apparatus is lowered and the production cost is raised. When a part of the current control circuit is constructed with a monolithic IC, therefore, the method using a capacitor is employed for preventing the oscillation. According to this method, however, the use of a capacitor will raise the cost of the apparatus and also increase the size of the apparatus.